1. Field of the Invention
The present invention relates to an ultrasound diagnostic apparatus, and more particularly to an ultrasound diagnostic apparatus having a continuous wave Doppler mode.
2. Description of Related Art
In a continuous wave Doppler mode, ultrasound (a continuous wave) is transmitted from an ultrasound probe into a living body, and a reflected wave (a continuous wave) subject to the Doppler effect due to the flowing of blood within the living body is received by the ultrasound probe. By applying frequency analysis to Doppler frequency components of a received signal, a Doppler waveform can be formed, as described in Japanese Patent Laid-Open Publication No. Sho 62-22637 (Reference 1).
Conventionally, in the continuous wave Doppler mode, a transmitting aperture and a receiving aperture are fixedly set on an array transducer provided in a probe, as described in Japanese Patent Laid-Open Publication No. 2004-159832 (Reference 2). With this structure, only a specific portion (i.e. the transmitting aperture) on the array transducer continuously generates heat, leading to a temperature gradient on a transmitting and receiving surface of the probe. For safety considerations, it is necessary that the temperature of the transmitting and receiving surface of the probe be maintained below a predetermined temperature. Accordingly, conventionally, when the temperature of the transmitting and receiving surface of the probe reaches the allowable limit, the power of the ultrasound to be transmitted is decreased. Such a decrease in the power of the ultrasound, however, reduces the sensitivity of the apparatus. Also, continuous use of a specific section of the array transducer as the transmitting aperture accelerates deterioration of that section, which reduces the operating life of the probe. Japanese Patent Laid-Open Publication No. 2004-057460 (Reference 3) describes setting of a transmitting aperture and a receiving aperture on an array transducer arranged two-dimensionally. In Reference 3, however, these apertures are arranged in a fixed manner.
Japanese Patent Laid-Open Publication No. Hei 10-75954 (Reference 4) discloses a structure in which the ratio of the size of a transmitting aperture and the size of a receiving aperture on an array transducer is variable in accordance with, the position of a measuring target (particularly, in accordance with the beam direction). Further, Reference 4 describes that when the measuring target is located toward the right with respect to the front of the probe, a transmitting aperture is set on the right side of the array transducer and a receiving aperture is set on the left side of the array transducer, whereas, when the measuring target is located toward the left with respect to the front of the probe, the transmitting aperture is set on the left side of the array transducer and the receiving aperture is set on the right side of the array transducer. In the structure described in Reference 4, however, if the position of the measuring target is unchanged (more specifically, unless the measuring target is changed toward right or left beyond the front position), the position of the transmitting aperture on the array transducer also remains unchanged. In addition, Reference 4 includes no description concerning countermeasures against heat generation or deterioration of the array transducer.
U.S. Pat. No. 6,669,638 (Reference 5) discloses a system for controlling the temperature of a transducer. While Reference 5 describes that a transmitting aperture and a receiving aperture are changed in accordance with the depth of the area being imaged, it does not describe that a transmitting aperture and a receiving aperture are changed in accordance with the temperature or that aperture patterns are switched.